Pesticides in Agricultural and Urban Streams of the United States.

Robert J. Gilliom, U.S. Geological Survey, Sacramento, California.

One of the main objectives of the National Water Quality Assessment (NAWQA) is to provide a national view of which pesticides occur most frequently in streams, at what concentrations, and what sources and land uses they are associated with. A key part of the NAWQA approach to addressing these questions is intensive monitoring of dissolved pesticides in streams within selected agricultural and urban settings and in associated major rivers draining large basins with mixed land uses. Preliminary results are available for 32 agricultural streams, 9 urban streams, and 8 major rivers in 17 NAWQA study areas throughout the nation. For most sites, more than 20 samples were collected in 1993, with the most frequent sampling during spring and summer. The national data set includes 1,070 samples analyzed for 48 dissolved pesticides, including many of the compounds with greatest current use. Method detection limits were less than 10 ng/L for most analytes. Nationally, all 48 pesticides were detected at least once, most samples contained more than one detectable compound, and 13 herbicides and 5 insecticides were detected in more than 10 percent of the samples. Drinking water (MCLs) and aquatic-life criteria established by the Environmental Protection Agency were not often exceeded, however. The most detected herbicides were atrazine (84% of samples), metolachlor (75%), simazine (63%), and prometon (53%). Atrazine was frequently detected at all types of sites, whereas important parameters of model sensitivity allows us to focus future efforts on collecting all available information on the given parameters and, consequently, to reduce their input uncertainty.